Surge arresters are used to protect electrical equipment, such as transformers and circuit breakers, in substations and in rail systems against the effects of overvoltages caused by incoming surges. Such overvoltages can be caused by a direct or nearby lightning strike, an electromagnetic pulse, electrostatic discharge, or switching operations in the power supply system as well as in devices. Some overvoltages are very high in energy. The current from the surge is diverted through the arrester, in most cases to earth. Effective overvoltage protection requires that different surge arrester types be used according to the particular application.
A voltage sag is a short duration reduction in voltage which can be caused by a short circuit, overload or starting of electric motors. Voltage sag happens when the voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute. A longer duration of low voltage would be called “sustained sag”. The term voltage sag should not be confused with brownout which is the reduction of voltage for minutes or hours.